1. Field of the Invention
The present invention relates to a variable magnification optical system and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system that is usable in a video camera, an electronic still camera, and the like, and which is appropriate especially for use in a monitor camera. Further, the present invention relates to an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, monitor cameras are used to prevent crimes, to record images, or the like. Optical systems for the monitor cameras need to be small and at low cost. The optical systems need to have large aperture ratios so that subjects are identified even in low-illumination photography conditions, as well as having high optical performance. Further, a demand for monitor cameras with variable magnification functions increased in recent years. Naturally, the main trend of optical systems for monitor cameras will be variable magnification optical systems. The variable magnification optical systems that can be used in monitor cameras are disclosed, for example, in U.S. Pat. No. 7,167,318 (Patent Document 1) and U.S. Pat. No. 7,050,240 (Patent Document 2). The variable magnification optical systems disclosed in Patent Document 1 and Patent Document 2 are two-group zoom optical systems, each composed of a first lens group having a negative refractive power and a second lens group having a positive refractive power. The variable magnification optical systems disclosed in Patent Documents 1 and 2 are six-lens or eight-lens optical systems as a whole, and all lenses in the first lens group are aspheric lenses.
Meanwhile, imaging devices, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), are mounted on cameras in the aforementioned fields. In recent years, the resolution of the imaging devices became higher, and a demand for high-quality images or video images by use of monitor cameras increased. Especially, a demand for high-performance variable magnification optical systems that can cope with imaging devices having pixel numbers of 1 million or higher is increasing. However, in conventional optical systems, it was difficult to achieve high-performance optical systems that can cope with the increase in resolution in recent years, while the large aperture ratio and the compactness, which are necessary for use in monitor cameras, are maintained.